The present invention relates to a method and apparatus for cleaning by spraying a cleaning fluid such as an aerosol containing argon particles toward a surface to be cleaned of an object to be cleaned such as a semiconductor wafer.
Particles or other contaminants on a surface of a semiconductor wafer in the LSI manufacturing process, and on a surface of a liquid crystal display (LCD), a solar cell, or the like, have a direct bearing on product yields, and therefore surface cleaning of wafers or the like is a crucial step.
Various surface cleaning methods have thus been devised. In the semiconductor manufacturing process, for example, wet bath cleaning methods have been most commonly used, in which objects to be cleaned are cleaned with pure water with the application of ultrasonic wave, or immersed in solutions containing cleaning agents (for example, hydrogen peroxide and ammonium hydroxide, or hydrogen peroxide and sulfuric acid) in pure water.
However, these wet bath-cleaning processes require an extensive site occupied by various pieces of equipment, and proper processing of disposed solutions. Consequently, there has been a desire to move toward a more environmentally friendly cleaning method without disposing any solutions or the like because of the recent increasing need of protecting the environment.
One of the dry cleaning methods, which don""t utilize liquids, utilizes chemical reactions by applying a gas, but contaminants in the form of particles can hardly be removed by this method.
Another dry cleaning method may involve colliding particles of dry ice, ice, or solid argon with a surface to be cleaned to remove contaminating particles. However, cleaning with ice particles has the risk of damaging the surface which is being cleaned, and dry ice poses the problem of secondary contamination, because most dry ice products available on the market are made from exhaust gas produced through steel processing or oil refining and the dry ice itself is often contaminated.
On the other hand, the surface cleaning methods disclosed in Japanese Patent Laid-Open Publication Nos. Hei. 6-252114 and Hei. 6-295895, in which an aerosol containing solid argon particles (referred to as xe2x80x9cargon aerosolxe2x80x9d) is collided with an object in a depressurized atmosphere, do not pose the above-described problems.
FIG. 1 is a piping diagram of the entire construction of one example of a wafer cleaning apparatus using the argon aerosol. FIG. 2 is a plan view of the same, and FIG. 3 is a longitudinal cross-section of a cleaning chamber.
Mass flow controllers 30, 32 are provided for respectively adjusting the flow rates of argon gas and nitrogen gas. The argon gas and nitrogen gas are passed through a filter 34 and cooled within a heat exchanger 38 using, for example, a helium (He) cryocooler 36, after which they are ejected into a cleaning chamber 42 for cleaning a wafer, as an aerosol 24 from a large number of fine orifices 22 formed in a cleaning nozzle 20. Inside the cleaning chamber 42 is created a vacuum by a vacuum pump 40.
The wafer 10 rests on a process hand 46, which is moved in directions along the X- and Y-axes by a wafer scan mechanism 44 (therefore is referred to also as an xe2x80x9cX-Y scan stagexe2x80x9d), so that the entire surface of the wafer can be cleaned.
It has been devised to provide an acceleration nozzle 56 for supplying a gas to increase the speed of the aerosol so as to enhance the cleaning effect. Thus, nitrogen gas (referred to accelerating gas 58) is supplied to the acceleration nozzle 56 through a mass flow controller 52 and a filter 54, and is blown out from nozzle orifices, accelerating the aerosol 24 ejected from the cleaning nozzle 20, as illustrated in FIG. 4.
It has also been suggested that nitrogen gas be introduced as a purge gas 66 into the cleaning chamber 42 through a mass flow controller 62 and a filter 64 from one end (left hand side of FIG. 2) of the cleaning chamber 42, so as to prevent particles that have been removed from being deposited again onto the wafer surface.
Reference numeral 50 in FIG. 3 represents a shield for controlling the gas flow within the cleaning chamber 42.
Cassettes 72 accommodating wafers 10 are loaded from the outside of the apparatus into cassette chambers 70, in which a vacuum is drawn. The cassette chambers 70 are provided in a pair as shown in FIG. 2 for the exchange of cassettes 72. Within a robot chamber 80 (or a transfer chamber) is installed a vacuum transfer robot (referred to also as a vacuum robot) 82, having a robot arm 84 and a robot hand 86 mounted at the distal thereof. The wafers 10 are transferred by the robot hand 86 through gate valves 74, 76 onto the above-mentioned process hand 46 within a buffer chamber 90, which is used for transferring the wafer 10 into the cleaning chamber 42.
The process hand 46 is driven by the wafer scan mechanism 44 to move the wafer 10 thereon from the buffer chamber 90 into the cleaning chamber 42, and in directions along the Y-axis and the X-axis under the cleaning nozzle 20.
The front surface of the wafer 10 is thus cleaned entirely by the aerosol 24 ejected from the cleaning nozzle 20. Thereafter, the cleaned wafer 10 is returned to the cassette chamber 70 through the buffer chamber 90 in reverse motions.
Meanwhile, the increasing demand for higher performance of semiconductor wafers in recent years has highlighted various problems. For example, there is the problem, which the conventional wet bath cleaning method cannot resolve, that the front side of the wafer may be re-contaminated by the contaminants or particles on the backside of the wafer transferring onto the front side of the wafer. Even the cleaning method using aerosol has the problem of particles that have been sputtered away from the front side of the wafer depositing onto the backside of the wafer.
The conventional aerosol cleaning method involves only the cleaning of the upper surface (or front surface) of the wafer 10 by spraying an aerosol 24 containing solid fine particles ejected from the cleaning nozzle 20 from above the wafer 10 downwards, as shown in FIG. 1. This is because the cleaning fluid is an aerosol, which contains microscopic particles of a solid or a liquid that are strongly affected by gravity. Thus it has not been proposed to clean the lower surface (or backside) of the wafer 10 using an aerosol.
When cleaning a semiconductor wafer with the above-described wafer cleaning apparatus using an aerosol, it is preferable that the aerosol 24 be delivered obliquely toward the downstream of the flow of purge gas as shown in FIG. 4, in the case in which no apertures such as via holes are formed in the wafer 10. However, when the wafer has via holes 12 as shown in FIG. 5 or other surface irregularities to be cleaned, the aerosol 24, if delivered obliquely, can hardly reach inside of etched concavities as shown in FIG. 5. In this case, therefore, it would be more preferable to direct the aerosol vertically as shown in FIG. 6.
On the other hand, when the number of residual particles on the wafer after cleaning matters most, the aerosol should be delivered toward the wafer at an inclined angle as shown in FIG. 4, so as not to disturb the flow of purge gas.
To solve the above-described problems, the ejection angle of the aerosol 24 could be varied by making the mounting angle of the cleaning nozzle 20 adjustable. This will, however, bring about the following problems:
(1) There will be limitations on the angle of ejection because of the liquid argon within the cleaning nozzle 20;
(2) The adjusting mechanism will be considerably complex because of the necessity to cool the cleaning nozzle 20 to a temperature as low as near that of liquid nitrogen; and
(3) Varying of the angle of the cleaning nozzle 20 during cooling will cause a change in the condition within the nozzle, resulting in unstable discharge of the aerosol 24.
The present invention has been devised to solve the above-described problems in the prior art, and it is a first object of the invention to enable removal of contaminants or particles from the underside of an object to be cleaned.
A second object of the present invention is to achieve cleaning of both the upper side and the lower side of an object to be cleaned at the same time, so that contaminants or particles sputtered away from the surface being cleaned will not be deposited on the opposite side of the object to be cleaned.
A third object of the present invention is to make the direction of cleaning fluid toward a surface of the object to be cleaned adjustable without changing the mounting angle of cleaning nozzle.
The present invention provides a method of cleaning by spraying a cleaning fluid to a surface to be cleaned of an object to be cleaned, wherein a lower side of the substantially horizontally-held object to be cleaned is cleaned by spraying the cleaning fluid upwards from below the object to be cleaned toward the lower side of the object to be cleaned, thereby achieving the above-mentioned first object.
The object to be cleaned may be a semiconductor wafer.
Also, the cleaning fluid may be an aerosol containing argon particles.
Furthermore, the cleaning fluid may be accelerated by an accelerating fluid before being sprayed onto the surface to be cleaned.
Also, the spraying direction of the cleaning fluid toward the surface to be cleaned may be changed by the accelerating fluid.
In addition, the accelerating fluid may be ejected at variable speeds and in variable directions for controlling the spraying direction of the cleaning fluid toward the surface to be cleaned.
The present invention further provides a method of cleaning by spraying a cleaning fluid to a surface to be cleaned of an object to be cleaned, wherein a lower side and an upper side of the substantially horizontally-held object to be cleaned are cleaned at the same time by spraying the cleaning fluid upwards from below the object to be cleaned toward the lower side of the object to be cleaned and by spraying the cleaning fluid downwards from above the object to be cleaned toward the upper side of the object to be cleaned, thereby achieving the above-mentioned second object.
The present invention further provides a method of cleaning by spraying a cleaning fluid to a surface to be cleaned of an object to be cleaned, wherein the cleaning fluid is accelerated by an accelerating fluid, and caused to change its spraying direction toward the surface to be cleaned, before being sprayed onto the surface to be cleaned, thereby achieving the above-mentioned third object.
To achieve the above-mentioned first aim, the present invention further provides an apparatus for cleaning by spraying a cleaning fluid to a surface to be cleaned of an object to be cleaned, the apparatus having means for holding the object to be cleaned substantially horizontally, and a cleaning nozzle for spraying the cleaning fluid upwards from below the object to be cleaned against a lower side of the substantially horizontally-held object to be cleaned.
The apparatus may further include an acceleration nozzle for accelerating the cleaning fluid ejected from the cleaning nozzle.
The apparatus may further include means for changing the speed and the direction of the accelerating fluid ejected from the acceleration nozzle.
To achieve the above-mentioned second object, the present invention further provides an apparatus for cleaning by spraying a cleaning fluid to a surface to be cleaned of an object to be cleaned, the apparatus having means for holding the object to be cleaned substantially horizontally, a lower side cleaning nozzle for spraying the cleaning fluid upwards from below the object to be cleaned to a lower side of the substantially horizontally-held object to be cleaned, and an upper side cleaning nozzle for spraying the cleaning fluid downwards from above the object to be cleaned to an upper side of the object to be cleaned.
To achieve the above-mentioned third object, the present invention further provides an apparatus for cleaning by spraying a cleaning fluid to a surface to be cleaned of an object to be cleaned, the apparatus having means for holding the object to be cleaned, a cleaning nozzle for spraying the cleaning fluid onto the surface to be cleaned of the object to be cleaned, and an acceleration nozzle for ejecting an accelerating fluid for changing the direction of the cleaning fluid ejected from said cleaning nozzle toward the surface to be cleaned.